1. Field of the Invention
The present invention relates to an oscillator and, more particularly, to an oscillator which is constructed to reduce deterioration in performance due to switching of different frequencies when transmission and reception are conducted with the different frequencies by using one oscillator.
2. Description of the Prior Art
In case wireless communications are to be conducted by means of one oscillator such as an oscillator using a micro-strip line as a resonance line, there has been used in the prior art a local oscillation frequency with which a shortened resonance circuit of 1/2.lambda. type having a resonance line equipped at its two terminals with capacitors C.sub.1, C.sub.2, C.sub.01, C.sub.02 and so on is oscillated by means of a transistor TR, as shown in FIG. 4. The oscillator constructs the so-called "voltage-controlled oscillator (VCO)" in which an oscillation frequency to be transmitted in the form of a shown output signal is subjected to a voltage control by the voltage level of a shown controlling input signal to be fed to a varactor BD.sub.1. In order that the oscillation frequency may be shifted to correspond to either of the frequencies for the transmission or reception, moreover, the resonance line 1 is loaded in parallel with the capacitor C.sub.2 to change the load capacitance by applying a forward voltage to a diode D.sub.1 to render it conductive with a shown shifting input signal.
With the construction thus made according to the prior art, as shown in FIG. 4, the load capacitor C.sub.1 is connected or not in parallel with the righthand terminal of the resonance line 1 in response to the conduction or inconduction of the diode D.sub.1. As a result, there arises a defect that the variable value of the frequency by the shown varactor BD.sub.1 is changed. If, at this time, the value Q of the resonance circuit is to be increased, it is necessary to reduce the characteristic impedance of the resonance line 1 to a small value. This necessity raises a problem that the load capacitance is further increased together with the influences of the above-specified defect. If the capacitance of the load capacitor connected with the terminal of the resonance line 1 varies, as shown in FIG. 5, the shown node point shifts to raise other problems that the level of the output to be extracted is accompanied by a difference and that the oscillating condition fluctuates in accordance with the shift of the node point to make it difficult to attain the optimum oscillating condition. If the characteristic impedance is reduced to increase the value Q of the resonance line 1, moreover, the load capacitor C.sub.2 has its capacitance increased so that the value of the high-frequency current to flow through the switching element (e.g., the diode) is increased to make the resistance R.sub.s innegligible for the SW-ON thereby to raise a further problem that the value Q of the resonance circuit cannot be increased.